1. Field of the Invention
The present invention relates to a disc driving apparatus and, more particularly, to an apparatus capable of properly positioning an optical head so that, when the optical head starts its operation of recording an information signal on or reproducing the same from a disc type recording medium, a spot of a signal recording/reproducing light beam emitted from the optical head to the disc is placed exactly at a predetermined position on in the innermost region of the disc.
2. Description of the Prior Art
On an optical disc known as a compact disc or the like, it is generally customary that a lead-in area is formed in an innermost region thereof where data (TOC) representing the table of contents of information recorded on the optical disc is described. At the start of a playback operation, an optical disc apparatus first reads out the TOC data from such innermost lead-in area. In this case, it is necessary to place the optical head at a predetermined position by moving the same to the innermost region.
FIG. 1 is a partially sectional side view showing the principal structure of a conventional optical disc apparatus, and FIG. 2 is a circuit block diagram of such conventional optical disc apparatus. In these diagrams, an optical disc 1 is loaded on a turntable 2 by means of a chucking device 3 in a manner to be rotatable by a spindle motor 5, which is anchored to a projecting center portion 4a of a chassis 4. An optical head 6 is movable on a guide shaft 7 supported between the center portion 4a of the chassis 4 and a projecting peripheral portion 4b of the chassis 4. A limit switch 11 is attached to a stand 12 anchored to a bottom 4c of the chassis 4 at a position proximate to the innermost region of the optical disc 1 (proximate to the spindle motor 5).
At the start of playback of the optical disc 1, a CPU 15 first transmits a command to a drive circuit 16 for reading out the data from the lead-in area, thereby supplying a predetermined voltage to a feed motor 8. Then the feed motor 8 is rotated by such voltage. Subsequently a speed reduction mechanism 9 is driven with rotation of the feed motor 8, and a rack gear 10 engaged with the optical head 6 is driven by the action of the reduction mechanism 9, so that the optical head 6 is moved on the guide shaft 7 toward the innermost region (leftward in FIG. 1). Upon completion of the shift of the optical-head 6 to the predetermined position in the innermost region, a sensing lever 13 of the limit switch 11 is pressed by the optical head 6 and therefore the limit switch 11 is closed, so that the CPU 15 detects the arrival of the optical head 6 at the predetermined position. Then the CPU 15 cancels the command to the driving circuit 16 synchronously with the closing of the limit switch 11, thereby halting the optical head 6.
In the conventional optical disc apparatus mentioned above, there exist the following problems. [1] An intricate procedure is necessary for executing a radial fine adjustment of the stand 12 where the limit switch 11 is attached, and turning on the limit switch 11 properly so that the spot 14 of the signal recording/reproducing light beam emitted from the optical head 6 at the stop position can be irradiated exactly to the lead-in area. [2] The speed of the optical head 6 at the time of pressing the sensing lever 13 is different depending on whether the position of the optical head 6 prior to its motion toward the innermost region is proximate to the limit switch 11 or is spaced apart therefrom, and consequently the distance of the inertial motion of the optical head 6 becomes different in such two cases. As a result, the beam spot 14 may fail to be irradiated accurately to the lead-in area even after the adjustment in [1].
[3] If the spot 14 of the signal recording/reproducing light beam emitted from the optical head 6 is deviated from the lead-in area, there may be adopted a method of returning the beam spot 14 to the lead-in area on the basis of the address information of the signal recorded at the deviated position. However, on any disc where the lead-in area is formed to be extremely narrow and no track is existent inside the lead-in area, it is impossible to apply tracking servo control at such position so that there is a failure in reproducing the signal. Therefore the beam spot is not returnable to the lead-in area.